1. Field of the Invention
This invention relates to a process for the digestion of animal by-products and the subsequent production of a concentrated protein product therefrom.
2. General Discussion of the Prior Art
Animals intended to be processed into food contain large amounts of protein, not all of which is often utilized. For example, in the production of fish fillets, a substantial amount of waste is left after removing the fillets from the fish. This waste is in the form of "fillet frames" comprising the head and tail, backbone, skin and fins, entrails, and leftover flesh not removed in the fillet. Also, large-scale fishing operations result in the capture of large numbers of "trash" fish which represent a valuable source of animal protein that is often wastefully discarded. Abattoir processing of poultry and large terrestrial animals for use as food yields a certain amount of waste material and by-products containing protein that would be beneficial to recover. Terrestrial animals include ovines (sheep), porcines (hogs), and bovines (cattle). Although fillets and conventional cuts of meat represent a more desirable form of animal protein for human consumption, the protein residing in "waste" parts also represents a valuable food source, particularly as animal food or food supplements.
In the interest of food production to either directly or indirectly support an expanding world population, it is economically and environmentally desirable to recover waste protein. Major uses for such protein include livestock feed and feed supplements, aquaculture feed, food flavor enhancers, and protein supplements for use in human food.
Many attempts have been made to recover protein from waste animal parts and by-products. For example, U.S. Pat. Nos. 4,361,586 to Meinke; 4,293,571 to Olofsson et al.; 4,176,199 to Vollmer et al.; 3,970,520 to Feldman et al.; 3,928,630 to Perini; 3,796,811 to Huth et al.; and Russian Patent No. 441,915 disclose methods by which animal by-products are enzymatically digested (hydrolyzed) to produce concentrated protein products. However, the methods disclosed in these patents have particular disadvantages. Many require long reaction times and are not adaptable to large-scale operations. For example, the methods of U.S. Pat. Nos. 4,361,568 and 3,928,630 require up to 24 hours for hydrolysis. The method of U.S. Pat. No. 4,176,199 requires four to six hours for digestion. In addition, the above-cited methods do not address how to monitor and control the troublesome problem of clogging of the processing machinery, other than by processing only relatively dilute suspensions of proteinaceous material. Such dilute suspensions require large amounts of energy to dehydrate after enzymatic digestion, which adds appreciable cost.
With the high cost of fuel and the need to conserve our natural resources, it is extremely important, especially in large-scale methods for processing animal by-products, that energy consumption be kept to a minimum. As large amounts of energy are required to remove water from high-moisture products, this need is in direct conflict with the need to keep high-protein products from spoiling by drying them to a point where bacteria and molds will no longer grow.
All of the above-cited methods that yield a dry product do so by utilizing conventional dryers which use heated air as a drying medium and are therefore relatively energy inefficient. For example, a spray dryer typical in the art requires 1600 to 1800 BTU's to remove a pound of water as compared to the 500 to 600 BTU's required for a double-effect evaporator to remove a pound of water. Some processes utilize evaporators to remove part of the moisture in the hydrolyzed protein but still must depend on the less efficient conventional dryers to bring the product to its desired dryness. This is because hydrolyzed protein can become very sticky and gummy and difficult to handle during an evaporation process.
Another problem with conventional drying methods is that they are a major source of environmental off-odors associated with conventional protein meal processes. Along with the release of moisture from a dryer, many compounds that cause objectionable odors, such as amine compounds, as well as particulate matter are also released into the atmosphere from conventional drying methods.
The odor problem is further complicated by the fact that many of these odoriferous compounds will react with the oxygen and nitrogen in the heated air and produce even more objectionable products.
Still another drawback to use of conventional dryers is that they are very large and expensive.
It is also important, particularly in large-scale, continuous methods for processing animal by-products, that the method be resistant to clogging of equipment. Clogging is a common problem because protein suspensions exposed to excessive heat and turbulence can undergo appreciable denaturation (analogous to cooking an egg) unless preventative measures and proper equipment design are employed. Even in the absence of denaturation, fat solids are commonly implicated in clog formation because the protein-digesting (proteolytic or protein-hydrolyzing) enzymes do not attack fats. Grinding the raw animal parts as a preliminary step to digestion yields no assurance that clogging will not occur in part because the ground material can easily reagglomerate. Clogging not only forces a shut-down of the processing machinery, but also results in significant waste of the proteinaceous material that must be removed from the equipment in order to restore operation. Further, equipment down-time as a result of clogging results in significant economic inefficiency which can make certain processes economically unfeasible. Further, an unanticipated clog can seriously erode one of the expected benefits of a continuous process; namely, the expectation that the process be able to run for significant periods of time without human intervention.
Many relevant processes known in the prior art include a similar series of steps. These include some form of grinding step; a digestion step wherein the animal protein is mixed with either endogenous or exogenous proteolytic enzymes to digest the raw animal protein; a step in which non-digestible solids are removed, such as bones, scales, and cartilage; and a drying step wherein at least a portion of the residual water in the digested material is removed. The steps during which clogging is most likely to occur are the removal of non-digestible solids and the drying step. However, clogging may occur virtually anywhere in the machinery. Clogs and agglomerates can be extremely difficult to remove and can cause equipment shut-down in a remarkably short period of time.
Clogging is often a problem with existing processes for other reasons. For example, such processes often include a step in which the protein suspension is partially cooked or strongly agitated either before or during enzyme digestion which causes appreciable protein denaturation. Also, other processes that utilize dilute suspensions in an effort to reduce clogging aim to remove the large amounts of water from the material after digestion by various drying steps wherein the resulting protein product becomes very stiff and difficult to pass through downstream machinery afterward.
Another disadvantage of certain existing processes is that they must be performed in batches wherein a given quantity of animal by-products after grinding is placed in a vat or the like containing enzymes, where the mixture is stirred for a given period of time before removal of non-digestible solids and drying of the product Such batch processes are inherently inefficient, are slower than continuous processes, and are consequently less economical than continuous processes.
Hence, there is a need for a method for producing a non-denatured, partially hydrolyzed protein product by a continuous process that is resistant to clogging.
Further, there is a need for such a process whereby the protein product produced thereby is in a particulate form for ease and longevity of storage.
Further, there is a need for such a process that yields a protein product wherein the degree of protein hydrolysis has been carefully controlled, resulting in a higher-quality protein for food use.
Further, there is a need for such a method yielding a product that can be left either in particulate form or compressed into pellets or blocks as needs or conditions dictate.
Further, there is a need for such a process yielding a product containing sufficient oil for desirable flavor characteristics and consistency without having an objectionable odor.
Further, there is a need for a protein product which contains high-quality oil for caloric value.
Further, there is a need for such a process yielding a protein product from which most of the residual water has been removed to enhance long term storability of the product without spoilage.
Further, there is a need for a process which economically removes most of the residual water from the protein product, even in large-scale operations.
Further, there is a need for a process which removes most of the residual water from the protein product without fouling the air with unpleasant odors and other undesirable compounds.